1. Field of the Invention
This invention relates to improvements in an apparatus for providing uniform contact of a downwardly moving, compact bed of particulate solids with a liquid which floods the bed. More particularly, the invention relates to an improved levelling apparatus to insure consistency and proper distribution of particles delivered and moved through the vessel; and to an arrangement for weighing the contents of a tank.
2. Prior Art
In many processes it is desired to contact particulate solids with liquids for a given period of time in order to accomplish a desired chemical or physical modification of the solids. When large amounts of solids are to be treated, it is most advantageous that they be disposed as a relatively compact bed in order to contain the solids in the smallest possible vessel and thereby save the considerable expense that the use of larger vessels would entail. In the compact bed it is desirable that there be little, if any, relative movement of the particles with respect to each other and, in the type of treatment involved, there is no need to provide agitation or intimate mixing of the liquid and solids since mere flooding of the bed with the treating liquid will suffice to accomplish the desired result.
These treatments of solids have often been accomplished in the past merely by providing a number of soaking tanks in which the solids are placed and then flooded with the liquid treating agent. After a period of time, the tanks are emptied and the operation repeated. This type of operation in a batch mode requires a plurality of treating tanks, allotment of periods for loading and unloading, and excessive labor cost. It is more desirable to conduct the operation on a more continuous basis to reduce the vessel capacity required and lower operating expense.
One approach to a more continuous type of treatment has been to feed the solids into the top of a treating vessel containing the treating liquid and remove treated solids from the bottom, and, in doing so, it is desired that the solids be disposed as a compact, downwardly moving bed in order to utilize a vessel of relatively small capacity for the amount of solids to be processed. Although appearing quite simple, difficulties in this operation do arise. In order to obtain a uniform length of contact between the solids and the treating liquid, any given solid particle should have approximately equal residence time in the liquid. To accomplish this goal, the compact bed should move relatively uniformly, downwardly through the vessel across substantially the entire cross-section of the vessel. This type of flow has often been characterized as plug flow and indicates the substantial absence of flow channels through the bed wherein some particles move faster than in other portions of the bed.
It is known to provide a vertical soak tank to accomplish relatively uniform, gravity flow, of the compact bed of particulate solids through a vessel containing the solids in contact with a liquid which floods the bed to fill the interstices between the solids over a substantial height of the vessel, for example in the commonly owned application of Wisdom et al., Ser. No. 972,404 filed Dec. 22, 1978. Such tanks are especially useful in contacting the solids with a liquid treating agent in order that the solids passing through the vessel have a relatively uniform residence time in contact with the liquid thereby insuring that given portions of the solids will not be treated for materially greater or lesser times than desired. As an example, the treatment of corn solids with an aqueous solution of calcium hydroxide can be accomplished in a continuous manner in such tanks at relatively uniform residence times. Such treatment facilitates the further processing of the corn without having to unduly adjust subsequent treating operations as might be required if the products withdrawn from the treating vessel were not subjected to relatively uniform treatment with the liquid.
In addition to problems noted above in obtaining plug flow, problems have also been encountered in initially distributing particles into the soak tank. One prior art approach involved feed pipes which simply dumped the corn or other particles into the vessel. This approach created mounds of particles beneath each pipe thus concentrating the particles in distinct areas with corresponding voids therebetween. As a result, the desired levelling effect is not achieved and the voids and concentrations may pass through the tank possibly creating different soak times for particular particles. In the Wisdom et al. application referred to above rotating equipment was utilized to distribute particles entering the vessel, somewhat successfully, but has required undue maintenance. Wisdom et al. shows a rotating perforated disc with a spreader arm positioned immediately above the disc for spreading particles thereover. Often the particles flowing and precipitation of solids of the liquid in and around this type of rotating equipment have caused clogging, jamming and otherwise impeded the movement of the spreader arm as well as other parts of the distribution apparatus. Substantial maintenance is required to keep the apparatus sufficiently clean and to avoid clogging, etc.
In determining the soak time it would be desirable to have the material being delivered to the vessel weighed such that an accurate accounting of the amount of material delivered is obtained. In the past this has been accomplished by weighing the material, or physically measuring the height of the corn bed, prior to the delivery; however, due to the unsymmetrical shape of the materials used, it is difficult to determine in advance exactly how much material the tank can hold particularly when it is compacted and levelled by the levelling system described above. Consequently, it is highly desirable to actually weigh the material once it is in the tank. This however presents problems due to the construction and contents of the tank which were previously thought to be impossible to overcome.